One of the major difficulties encountered in forming high quality nitride films is creating conditions in which sufficient nitrogen is incorporated in the film.
The material quality of the nitride can be significantly degraded if the fractional deficiency of nitrogen is as small as one part per hundred thousand. The difficulty arises because molecular gaseous nitrogen, N.sub.2, is chemically stable: in nitride deposition processes the formation of N.sub.2 is chemically favored over the formation of nitrides. In molecular beam epitaxy (MBE), for example, various schemes have been employed to deliver nitrogen to the substrate in chemically active forms--for example, by exciting gaseous nitrogen via plasma or electron cyclotron resonance sources, or by using ammonia (NH.sub.3) gas--to increase the likelihood that delivered nitrogen will react with the metal (e.g., with Ga to form GaN), somewhat overcoming the strong propensity of the delivered nitrogen to form N.sub.2.
There is a large technical literature pertaining to evaporative deposition of films. Long-established methods are summarized in the book by Joseph I. Goldstein et al., Scanning Electron Microscopy and X-Ray Microanalysis (New York and London: Plenum Press, 1992), Section 13.6, and in Chapter 9 of the book edited by S. M. Sze, VLSI Technology (McGraw-Hill, 1983). MBE has recently been reviewed in an article by K.-Y. Cheng, "Molecular Beam Epitaxy of III-V Compound Semiconductors for Optoelectronic Applications," Proceedings of the IEEE, Volume 85, No. 11 (November 1997), pp. 1694-1714. Use of MBE methods specifically for deposition of nitride films has recently been reviewed in the article by G. Popovici, H. Morko.cedilla., and S. N. Mohammed, "Deposition and properties of group III nitrides by molecular beam epitaxy," pp. 19-69 in the book Group III Nitride Semiconductor Compounds, edited by Bernard Gil (Oxford University Press, 1998).